In recent years, there have been widely used display devices, such as liquid crystal display devices, in which a backlight is used for illuminating a display panel from the rear or lateral side thereof. Among them, liquid crystal TVs and liquid crystal monitors employ fluorescent tubes, such as hot- or cold-cathode fluorescent tubes, in the backlights. The light-emitting devices such as fluorescent-tube backlights include a so-called direct-lighting type, in which fluorescent tubes are disposed in a plane directly beneath (on the rear side) a liquid crystal panel, and a so-called edge-lighting type, in which (a) fluorescent tube(s) is/are disposed along only two edges or one edge on a light guide plate made of transparent resins, and the incident light to the light guide plate is reflected on the reflecting portion disposed on the rear of the light guide plate to illuminate a liquid crystal panel.
The direct-lighting type has an advantage of securing high luminosity, but has a disadvantage of thick backlight structures caused by non-uniformity of the luminance of the fluorescent tubes. On the other hand, the edge-lighting type has an advantage of allowing thinner design than the direct-lighting type, but has a disadvantage of lower efficiency in light utilization. In view of these circumstances, as a substitute for the fluorescent-tube backlight, recently has been studied a backlight using a light emitting diode (LED), which is one of a solid-state light-emitting element, as a light source.
The use of LEDs can secure high luminosity at low electric power consumption in backlights for small-size displays such as liquid crystal displays for cellular phones. Such LED-backlights for liquid crystals have been so far used for small-size liquid crystal displays for cellular phones as described above, and nowadays such backlights are applied in wider fields covering from liquid crystal displays for car navigation systems to wide-screen TVs. In particular, attention is paid to backlights for wide-screen liquid crystal TVs composed of three types of LEDs corresponding to the three primary colors, red, blue, and green, because such backlights can reproduce wider range of color, thereby realizing high imaging quality.
In backlights for wide-screen liquid crystal TVs, it is required to array a number of LED chips in a direct-lighting configuration to meet the necessity of a large quantity of light. Currently, since the light-emitting efficiency (the conversion efficiency from electric energy to light energy) of LEDs is still 50% or less, hence over half of the energy input is transformed into heat. This heat elevates the temperature of LED chips, causing further decrease of the light-emitting efficiency and the shortening of lifetime of LEDs. Accordingly, it is desired to efficiently dissipate the heat generated by LEDs to the outside of the LEDs.
For example, Japanese Patent Application Laid-Open Publication No. 2006-11239 describes that, as a related art regarding LED light sources described in official gazettes, there is an art relating to an edge lighting-type backlight in which a mounted metallic film, a metallic wiring for driving, and a metallic film pattern are formed on the surface of a mounting substrate where an LED module is mounted, a heat-dissipating metallic film is formed on the rear of the mounting substrate, and the front and rear are connected via metallic through-holes to effectively suppress temperature elevation of the LED module itself.
Generally, LEDs are free from filament burning out, which is a problem of incandescent lamps, because a semiconductor itself emits light in LEDs. However, degradation of the LED element itself or the resin sealing the LED element decreases the light transmittance, thereby gradually lowering the luminosity. It is known that the degradation of the resin is accelerated by heat generated in the LED element. Particularly in LED-backlights for liquid crystal display device, a blue LED emitting shorter-wavelength light is often used, and such shorter-wavelength light, which has higher energy, further increases the degradation of the resin. If more driving current is supplied to the LED for keeping the display screen bright, the light intensity increases and the LED temperature becomes higher at the same time, thereby degrading the resin more severely.
In order to address the above problems, it could be possible that the heat dissipation of an LED-substrate is enhanced by attaching a heat-dissipating plate such as an aluminum plate to the rear of the LED-substrate, but the product price significantly rises, and hence this technology is not considered to meet the recent severe demand for cost reduction.
Furthermore, when LED-backlights are used for wide-screen liquid crystal TVs or wide-screen liquid crystal panels, it is preferred to dispose LEDs in a direct-lighting configuration to attain higher luminosity. In the direct-lighting system, directly beneath a liquid crystal panel, there are disposed plural units in each of which plural light emitting diodes for each of colors, red, green, and blue, are arrayed. On this occasion, it is not enough to take a measure for improving heat-dissipating efficiency in the single unit, and it is required to adopt heat-dissipating measure considering the neighboring units.